proposed patch https://www.redhat.com/archives/libvir-list/2014-January/msg00031.html

Now upstream.

commit 08d07e5fd8a4c072bf040b3949bbd969f98d1081
Author: Francesco Romani <fromani>
Date:   Thu Jan 16 17:11:15 2014 +0100

    spice: expose the QEMU disable file transfer option
    
    spice-server offers an API to disable file transfer messages
    on the agent channel between the client and the guest.
    This is supported in qemu through the disable-agent-file-xfer option.
    
    This patch exposes this option to libvirt.
    Adds a new element 'filetransfer', with one property,
    'enable', which accepts a boolean.
    Default is enabled, for backward compatibility.
    
    Depends on the capability exported in the first patch of the series.
    
    Signed-off-by: Francesco Romani <fromani>

commit 19bbc8127685d5c33252a9e2fd257d5ef5c3cb1b
Author: Francesco Romani <fromani>
Date:   Thu Jan 16 17:11:14 2014 +0100

    spice: detect if qemu can disable file transfer
    
    spice-server offers an API to disable file transfer messages
    on the agent channel between the client and the guest.
    This is supported in qemu through the disable-agent-file-xfer option.
    
    This patch detects if QEMU supports this option, and add
    a capability if does.
    
    Signed-off-by: Francesco Romani <fromani>

Verify this bug with libvirt-0.10.2-35.el6, The following was the verify steps

pkginfo
host:
rhel6.6
spice-server-0.12.4-8.el6.x86_64
libvirt-0.10.2-35.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.425.el6.x86_64


guest:
rhel6u5-64/win7-64
spice-vdagent-0.14.0-2.el6.x86_64/vdagent-win-3.3-2

client:
fc19
virt-viewer-0.5.6-1.fc19.x86_64

steps
1. start two guests with spice-agent enabled(it was enabled by default)
# virsh list
 Id    Name                           State
----------------------------------------------------
 40    rhel6m                         running
 44    win7                           running

2. Connect the two guests in the fc19 
#remote-viewer spice://$server_ip:$port_number
3.Drag a file from the fc19 to the guests, the file can be draged successfully
4.Destroy the two guests, then disable the spice agent in libvirt side
#virsh dumpxml rhel6m
# virsh dumpxml rhel6m |grep filetransfer -B 2
    <graphics type='spice' port='5903' autoport='no' listen='0.0.0.0'>
      <listen type='address' address='0.0.0.0'/>
      <filetransfer enable='no'/>

# virsh dumpxml win7 |grep filetransfer -B 1
    <graphics type='spice' autoport='yes'>
      <filetransfer enable='no'/>

5.start the two guests, check the qemu command of the guest, we could see the spice agent was disabled in qemu side
# ps aux|grep rhel6m 
qemu     21085 59.3  3.4 1511736 279984 ?      Sl   04:19   0:14 /usr/libexec/qemu-kvm -name rhel6m --
-device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -spice port=5903,addr=0.0.0.0,disable-ticketing,disable-agent-file-xfer,seamless-migration=on

#ps aux|grep win7
# ps aux|grep win7
qemu     21123 72.3 13.5 1593504 1084444 ?     Sl   04:19   1:05 /usr/libexec/qemu-kvm -name win7 --
-spice port=5900,addr=0.0.0.0,disable-ticketing,disable-agent-file-xfer,seamless-migration=on

6.Connect the two guests in the fc19 
#remote-viewer spice://$server_ip:$port_number

7.Drag a file from the fc19 to the guests, the file can't be draged to the guest successfully

According to the step 5, mark this bug verified

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2014-1374.html